The present invention relates to a metal corrosion prevention system using a non-sacrificial photoanode under sunlight, and more particularly to the metal corrosion prevention system using a photo-catalyst non-sacrificial photoanode containing titanium dioxide (TiO2) or zinc oxide (ZnO) and a hole scavenger (or water) to form a galvanic couple with the metal to be protected from the corrosion, thus the non-sacrificial can continuously provide electrons to the metal without consuming itself.
The metal corrosion prevention system of the present invention, which is a different system from conventional corrosion prevention systems, employs sunlight as an energy source, and is capable of being installed outdoors for an easy maintenance and being used semi-permanently. The photo-catalyst employed in the non-sacrificial photoanode according to the present invention converts photons into electrons and empty holes. In case that TiO2 is used as a photo-catalyst, the converted hole generates a hydroxyl radical (E∘=2.81 V ("ugr"s NHE)) having a high oxidation ability of various organic pollutants, which is widely used in advanced oxidation process (AOP), and the converted electron is used for the reduction of halogenated compounds and dioxin. In order to utilize the converted electrons, recombination between the converted electrons and the converted holes should be inhibited by eliminating the converted holes. For this purpose, generally an organic compound, which is called as a hole scavenger, is contacted with the photo-catalyst.
A conventional method for preventing the corrosion of a metal structure is to provide electrons to the metal structure for a cathodic protection. The cathodic protection is the method to restrain the anode reaction which incurs corrosion of the metal structure by artificially controlling electric potential. The cathodic process method to lower the electric potential by supplying electric current to the metal structure is generally used.
A sacrificial anode protection, which is one of the most well-known cathodic protection methods, prevents the corrosion of a metal structure by cathodizing the metal by connecting electrically with another metal having higher ionization probability than the metal to be protected within the electrolyte. This method is called as a sacrificial anode method because the metal having higher ionization probability is corroded (oxidized) and consumed. Aluminum and magnesium have been widely used as a sacrificial anode in the above-mentioned method.
However, the conventional sacrificial cathodic method has a disadvantage in maintaining the system because an anode should be frequently replaced due to the continuous corrosion.
An impressed current method, which is another well-known cathodic protection method, provides the protection of a metal structure from the corrosion by directly supplying electric current into the metal structure to be protected. In case that an external power supply is available in the near site, the impressed current method could be a useful method. Even though this protection method is suitable for a bigger structure than what a sacrificial cathodic method can protect, the installation of an additional power supply is inevitable.
Along with the rapid increase of study regarding solar energy, there are some prior applications, which utilize the solar energy in preventing the corrosion of metal. For example, U.S. Pat. No. H1644 disclosed the impressers method to prevent the corrosion of a metal structure by supplying directly, electricity generated from the solar energy by means of a photovoltaic solar cell (PV cell), which has been widely used in industries. The characteristic of this method is an impressed method which supplies the electricity generated from the photovoltaic solar cell to the metal structure to be protected by connecting general photovoltaic solar cell to the metal structure, and further an ultracapacitor is employed in case of night and overcastting weather. However, because this method has merely employed the conventional photovoltaic solar cell, the cost of initial installation and maintenance are considerable.
A cathodic protection can be divided into a voltaic-based protection and a current-based protection. The voltaic-based protection can be subdivided in accordance with base voltages of which a widely used voltage is xe2x88x920.85 V (vs. Cu/CuSO4) corresponding to xe2x88x920.74 V (vs. SCE). The current-based protection can be changed in accordance with surrounding circumstance of iron, and is usually 1 xcexcA/cm2 unless the circumstance is strong corrosive solution. However, things to consider in applying these criteria are the above-described criteria are only minimum protection conditions and it is not necessary to comply with all criteria at the same time, i.e. it is alternate minimum criteria. In other words, if one of criteria is satisfied, the system is determined to be in a protection state regardless of the other criteria.
A non-sacrificial photoanode according to the present invention comprises a photocatalyst such as titanium dioxide (TiO2) and zinc oxide (ZnO) which generates electrons and holes in response to light, whereby the electric potential of the photocatalyst will be moved to negative potential. When the photocatalyst is galvanically coupled with a metal, electrons move to the metal from the photocatalyst, and equilibrium electrode potential is achieved by the polarization of potential of the metal and the photocatalyst.
Because the anode according to a conventional art continuously provides electrons to the metal to be protected with consuming (corroding) itself, it is necessary to periodically replace the sacrificial anode with new one.
In order to overcome the above demerit, the present invention is provided with a photocatalyst used as a non-sacrificial photoanode, which can continuously provide the electrons in galvanic circuit without corrosion of the photocatalyst itself by converting photons into electrons in response to sunlight.
The photocatalyst has the attribute of a semiconductor as same as silicon. Moreover, the photocatalyst is more stable and inexpensive as well as it is not required sophisticated technique to manufacture unlike silicon.
Even though, since 1972, the study concerning the photocatalyst, especially titanium dioxide (TiO2), has been developed in the various fields such as the purification of a contamination, the decomposition of water, the photo-electrochemical solar cell containing dye sensitizer, and a super-hydrophilicity, the study or possibility to prevent the corrosion of a metal by means of a photocatalyst has not been reported yet.
Consequently, an object of the present invention is to suggest a new possibility and application of the photocatalyst as a non-sacrificial photoanode to prevent corrosion of a metal, and to develop an optimized non-sacrificial photoanode considering several variables including the density of sunlight.